This invention relates to test instruments for testing life expectancy of magnet wire used in dynamoelectric machines. While the invention is described with particular reference to the test instrument's ability to test magnet wire intended for use in dynamoelectric machines associated with inverter controls, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
It is well known that degradation of wire insulation can be caused by thermal, electrical, mechanical, or environmental stress, alone or in combination with one another. Thermal stress is introduced by aging, overloading and cycling. Electrical stress is related to dielectric, cracking, corona and transient spikes. Mechanical stress results from impact, stretching, and coil movement during coil winding, assembly, and machine operation. Environmental stress is introduced by moisture, chemical and foreign objects in the environmental area of the dynamoelectric machine. All of these stresses can be, and often are, present at the same time in applicational use. Motor manufacturers have long recognized the effects these factors have on magnet wire.
With the increased use of pulse width modulated (pwm) type of adjustable speed drives in the motor industry, the incidents of premature failure in wire insulation has increased. Dynamoelectric machines being controlled with pulse width modulated variable speed drives no longer experience the traditional sine wave voltage, which is a steady state condition with a maximum and a rms value. Rather, the machines experience a pulse width modulated voltage with significance harmonics and transients. We have found that motor failures in inverter applications result in premature failure of the wire insulation. To understand the failure mechanism of magnet wire insulation under such condition, we have devised a test instrument which simulates inverter output in order to evaluate wire performance in that environment.
The basis of the thermal aging model is a well known Arrhenius chemical reaction rate model: Life=A.times.e.sup.-(BT), where A and B are the constants determined by the activation energy in the reaction rate of the particular degradation reaction, and T is the absolute temperature. This model neglects threshold effects (aging starts only if a certain temperature is reached), and the model assumes that only one chemical reaction is occurring. Nevertheless, this model presently is an accepted standard in industry.
Voltage aging models also have been developed. The voltage aging models conventionally are divided into two groups, the inverse power model and the exponential model. The inverse power model of electrical aging is expressed as: Life=K.times.E.sup.-n, where K is a constant and a characteristic of the material system, n is an experimentally obtained constant depending on operating conditions, and E is the applied voltage.
An alternative, exponential life model is given by: Life=C.times.e.sup.-DE, where C and D are constants, and E is the value of the applied voltage.
The basic models, given by the above equations, have been changed in order to allow for the description of threshold degradation, or combined in order to provide a model for multi-factor aging.
We were aware of the research being conducted in this area. Motivated by the number of early failures occurring in industrial applications of power inverter drives, we have designed a testing system that allows for the varying of the parameters that are characteristic of power inverters, i.e., voltage, frequency, duty cycle, and rise time in combination with the conventional thermal aging problem inherent in insulation dialectics applied to magnet wire, or other portions of the insulative system applied to a dynamoelectric machine.
One of the objects of this invention is to provide a test instrument which permits improved testing of magnet wire intended for use in dynamoelectric machines.
Another object of this invention is to provide a test instrument which tests wire which is intended for use in applications having pulse width modulated drive systems.
Another object of this invention is to provide a test instrument which permits variations of pulse width, voltage, rise time and temperature applied to the test sample.
Another object of this invention is to provide a test instrument which permits testing of both voltage and/or current variation and a test sample.
Another object of this invention is to provide a test instrument which permits easy modification of the testing procedure for implementing additional factors which may affect insulation life.
Another object of this invention is to provide a test instrument which is relatively easy to operate.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.